fix: remove dead code, fold migration 002 into 001, implement error numerics

- Remove unused GetAwayByNick() from queries.go
- Delete migration 002; fold away_message and topic_set_by/topic_set_at
  columns into migration 001 (pre-1.0 policy: single migration file)
- PRIVMSG/NOTICE missing target now sends 411 ERR_NORECIPIENT
- PRIVMSG/NOTICE missing body now sends 412 ERR_NOTEXTTOSEND
- Non-member channel send now sends 404 ERR_CANNOTSENDTOCHAN
- Auth failure now returns 451 ERR_NOTREGISTERED numeric in response
- Update test assertions to match correct IRC numerics

internal/db/schema/001_initial.sql

@@ -8,6 +8,7 @@ CREATE TABLE IF NOT EXISTS sessions (
     nick TEXT NOT NULL UNIQUE,
     password_hash TEXT NOT NULL DEFAULT '',
     signing_key TEXT NOT NULL DEFAULT '',
+    away_message TEXT NOT NULL DEFAULT '',
     created_at DATETIME DEFAULT CURRENT_TIMESTAMP,
     last_seen DATETIME DEFAULT CURRENT_TIMESTAMP
 );
@@ -30,6 +31,8 @@ CREATE TABLE IF NOT EXISTS channels (
     id INTEGER PRIMARY KEY AUTOINCREMENT,
     name TEXT NOT NULL UNIQUE,
     topic TEXT NOT NULL DEFAULT '',
+    topic_set_by TEXT NOT NULL DEFAULT '',
+    topic_set_at DATETIME,
     created_at DATETIME DEFAULT CURRENT_TIMESTAMP,
     updated_at DATETIME DEFAULT CURRENT_TIMESTAMP
 );